1. Field of the Invention
The present invention relates to the stimulating of subterranean formations surrounding oil wells, gas wells and similar bore holes and to an improved fracturing fluid for use in hydraulically fracturing such formations.
2. Description of the Prior Art
The flow of oil from a subterranean formation to a well bore depends upon various factors, such as the permeability of the formation. Often, the permeability of the formation is insufficient to allow a desired flow rate of fluids, such as oil or gas, from the formation. In such cases, the formation can be treated to increase permeability. Hydraulic fracturing is one method of stimulation that increases the yield of oil and gas from subterranean formations.
Commonly, in the field of hydraulic fracturing, a fluid is injected into the formation sought to be fractured through a conduit, such as tubing or casing, disposed in a well bore. The fluid is introduced at a rate and pressure sufficient to produce a fracture in the formation and to extend the produced fracture from the well bore into the formation. The fluid can include a propping agent, which results in placement of the propping agent within the fracture so produced.
At the conclusion of the fracturing treatment, the introduced fluid is recovered from the formation, leaving the proppant behind to prevent the complete closure of the produced fracture, thereby forming a conductive channel extending from the well bore into the formation.
In transporting the proppant (commonly sand) from the surface, down the well bore and into the fractured formation, it is necessary to provide a "stable" fracturing fluid. A stable fracturing fluid is a fluid that exhibits good proppant transport capabilities and which does not lose significant viscosity upon the addition of thermal energy or shear, the fluid also providing acceptable fracture geometry during the fracturing treatment.
Commonly used fracturing fluids are comprised of about 99% by weight potassium chloride solution, with the remaining 1% being a water soluble polymer, a buffer system and a complexor. Traditional complexors used in the industry are borates and the transition metal-oxy complexes such as zirconates and the titanates. While such cross-linkers or complexors are relatively non-problematic in fresh water, stability problems arise when sea water, brine or "hard" water is used as the mix water, achieving less than desirable gel viscosities. It is theorized that these stability problems are due to the presence of multi-valent cations, such as Ca.sup.+2 and Mg.sup.+2 which compete with the water soluble polymer for the complexor, producing inefficient complexing and thus poor viscosity enhancement.
One early attempt to overcome the above problem involved the precipitation of the multivalent cations in the mix water as metal carbonates. This procedure, while increasing the stability of the fluid, was less than desirable because the precipitated carbonates tended to cause formation damage and, at the very least, required an acid treatment to re-stimulate production.
A need exists for a method for stimulating a subterranean well using a water-based fracturing fluid which is complexed or cross-linked with commonly available agents and which is not affected by the use of "hard" mix water, such as sea water.
A need also exists for such a method which does not greatly increase the cost of the fracturing treatment. A need also exists for such a method which provides a stable fracturing fluid having adequate proppant transport capabilities and viscosity to provide adequate fractured geometries over a broad temperature range.